Travelling wave tube with two stage electron concentrating and focusing



Feb. 28; 1967 v P. DELPHIN 3,307,064 TRAVELLING WAVE TUBE WITH TWO STAGE ELECTRON CONCENTRATING AND FOCUSING Filed Dec. 11, 1962 N. Q L1... L0

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United States Patent Office 3,307,064 TRAVELLING WAVE TUBE WITH TWO STAGE ELECTRGN CON CENTRATING AND FOCUSING Pierre Delphin, 79 Blvd. Haussmann, Paris, France Filed Dec. 11, 1962, Ser. No. 243,940 Claims priority, application France, Jan. 11, 1962, 884,505, Patent 1,318,760 2 Claims. (Cl. 315-35) The present invention relates to travelling wave tubes of the O-type having a low noise factor, and more particularly to those tubes intended to operate at centimetric or millimetric wave-lengths.

It is known that within the tubes of this type, in which the electron beam, strongly focused, propagates along the axis of the tube on the inside of a delay line, constituted most frequently by a helix, the radius a of the helix must satisfy the following relation:

Ag is the guided wave length, x is the wave length in free space, is the speed of light and v is the speed of propagation of the delayed wave.

This condition, necessary to obtain a satisfactory coupling between the beam and the wave propagated by the delay line, requires a reduction of the diameter of the helix with the wave length or band of wave lengths utilized.

Thus, in the case of centimeter waves or particularly millimeter waves, the diameter of the helix must be very small and the beam must have a very small transverse cross section.

If, under these conditions, one desires to utilize a cathode having an emissive surface of an area equal to that of the transverse cross section of the beam propagating on the inside of the helix, as is usually the case in the conventional models of the tubes in question, the electron current supplied by the cathode will have a very small value, even with the highest emission densities practically realizable.

On the other hand, one would not envisage the use of a cathode of large emissive surface with a gun causing the emitted electron flow to converge into a very small beam in the vicinity of the cathode, for it is known that this type of gun increases the temperature of the beam and consequently the noise thereof.

The present invention has as its object an improved tube capable of operating at centimeter or millimeter wave-lengths without the inconveniences mentioned hereinabove.

According to the present invention, a travelling wave tube of the O-type, with low noise, for centimetric or millimetric waves, in which an electron beam, issued from an emissive cathode, is coupled to a delay line, the beam being focused by a magnetic focusing field directed along the axis of the tube, is characterized by the fact that the magnetic field is adjusted to furnish a focusing such that the transverse cross section of the beam is substantially equal to the emissive surface of the cathode over the portion of the beam extending between the cathode and the delay line, and is less than this surface over the portion of the beam extending facing the delay line, whilst the passage or transition from one portion to the other portion is realized by the progressive variation of the transverse cross section of the beam.

Accordingly, it is an object of the present invention 3,307,064 Patented Feb. 28, 1967 to provide an O-type travelling wave tube which eliminates, by simple means, the shortcomings encountered with the prior art constructions.

It is another object of the present invention to provide an O-type travelling wave tube capable of operating at centimeter or even millimeter wave lengths with relatively high power and low noise.

Still another object of the present invention resides in the provision of an O-type travelling wave tube which permits a relatively large emissive surface of the cathode notwithstanding the relatively small cross sectional dimensions of the interaction space defined by the delay line.

A further object of the present invention resides in the provision of a high power low noise O-type travelling wave tube operable at relatively high frequencies, that is, at centimetric or millimetric wave-lengths, with relatively high current densities of the beam.

These and other objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing in which FIG. 1 shows, for purposes of illustration only, one embodiment of a tube in accordance with the present invention, and FIGURE 2 shows the corresponding magnetic field along the axis of the tube.

Referring now to FIGURE 1 which illustrates schematically, in longitudinal cross section, a travelling wave tube in accordance with the present invention, reference numeral 1 designates therein an evacuated enclosure. The travelling wave tube in accordance with the present invention comprises, on the inside of the evacuated enclosure 1, an electron gun 2, a delay line constituted by a helix 3, and a collector electrode 4.

The electron gun 2 comprises a cathode 2 and various accelerating and control electrodes, and the cathode has a circular emissive surface of a diameter larger than that of the helix 3.

Outside of the tube is placed a focusing system comprising permanent magnets 5 having pole pieces 6 and a field straightener 7, constituted by washers alternately made of soft iron and dielectric material, bonded to each other.

All of the elements constituting a travelling wave tube described hereinabove are, per se, well-known in the art so that a detailed description of the operation thereof is dispensed with herein.

The essential characteristic of the present invention resides, for the described embodiment, in the particular arrangement of the field straightener 7 which is provided, as illustrated in the figure, with more closely spaced magnetic washers along the portion surrounding the electron gun than along that portion surrounding the helix, wherein the spacing may vary progressively along the passage or transition from one portion to the other.

Under these conditions, the magnetic field, intended to focus the electron beam, has a greater intensity along the helix than along the gun, and for well determined values of these intensities the electron beam, indicated in dash lines in the drawing, has a transverse cross section equal at first to the surface of the cathode 2' and later on slightly less than the transverse cross section of the helix 3.

FIGURE 2 shows the course of the magnetic induction B along the electron beam access V. It is seen that the induction B along the helix 3 is higher than the induction B along the electron gun, and that the variation from E to B is progressive along a short portion of the tube.

This arrangement therefore permits the utilization of a cathode having a diameter larger than that of the helix whereby the latter may be very small in those tubes which operate at centimetric or especially millimetric wave lengths.

While I have shown and described one embodiment in accordance with the present invention, it is understood that the same is not limited thereto, but is susceptible of numerous changes and modifications as known to a person skilled in the art. For example, instead of the permanent magnet, the focusing system may also comprise solenoids in which case, within the spirit of the present invention, the windings of the coils or the currents which energize the same, are disposed in such a manner that the number of ampere-turns is greater about the helix than about the rest of the tube.

Thus, while I have shown and described one embodiment in accordance with the present invention, it is understood that the same is not limited thereto, but is susceptible of numerous changes and modifications within the spirit and scope thereof, and I therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

I claim:

1. A low-noise, O-type traveling wave tube for centimeter and millimeter waves, comprising:

an electron gun disposed within a first portion of the tube for emitting an electron beam having a predetermined initial cross section,

a substantially helicoidal wave delay line disposed within a second portion of the tube about the axis thereof,

the diameter of the helix being less than the diameter of said initial cross section,

and focusing means operable to provide magnetic lines of force substantially parallel to the axis of the tube with such intensities that the initial cross section of the electron beam is maintained substantially along said first portion of the tube and is reduced within the second portion thereof to a diameter somewhat less than the diameter of the helix,

said focusing means including a magnet structure and field-straightener means composed of dielectric washers alternating with ferromagnetic washers, the distance between consecutive ferromagnetic washers being smaller in the said first portion of the tube than in the said second portion thereof.

2. A low-noise, O-type traveling wave tube for centimeter and millimeter waves, comprising:

an electron gun means located within a first portion of the tube for emitting an electron stream having a predetermined initial cross section,

a Wave guiding structure disposed within a second portion of the tube and defining at least in part the cross sectional area of an interaction space,

the cross sectional area of said interaction space being less than said initial cross section,

and magnetic focusing means operable to provide magnetic lines of force with such intensities that the initial cross section of the electron stream is maintained substantially along said first portion of the tube and is reduced within the second portion thereof to a cross section somewhat less than the cross section of said wave guiding structure,

said magnetic means including field-straightener means having dielectric washers alternating with ferromagnetic washers, the distance between consecutive ferromagnetic washers being smaller in the said first portion of the tube than in the said second portion thereof.

References Cited by the Examiner UNITED STATES PATENTS 2,840,752 6/1958 Cutler et al. 3l53.5 X 3,092,745 6/1963 Veith et al 3l53.5 X 3,141,992 7/1964 Delphin 3153.5

ELI LIEBERMAN, Primary Examiner.

R. D. COHN, Assistant Examiner. 

1. A LOW-NOISE, O-TYPE TRAVELING WAVE TUBE FOR CENTIMETER AND MILLIMETER WAVES, COMPRISING: AN ELECTRON GUN DISPOSED WITHIN A FIRST PORTION OF THE TUBE FOR EMITTING AN ELECTRON BEAM HAVING A PREDETERMINED INITIAL CROSS SECTION, A SUBSTANTIALLY HELICOIDAL WAVE DELAY LINE DISPOSED WITHIN A SECOND PORTION OF THE TUBE ABOUT THE AXIS THEREOF, THE DIAMETER OF THE HELIX BEING LESS THAN THE DIAMETER OF SAID INITIAL CROSS SECTION, AND FOCUSING MEANS OPERABLE TO PROVIDE MAGNETIC LINES OF FORCE SUBSTANTIALLY PARALLEL TO THE AXIS OF THE TUBE WITH SUCH INTENSITIES THAT THE INITIAL CROSS SECTION OF THE ELECTRON BEAM IS MAINTAINED SUBSTANTIALLY ALONG SAID FIRST PORTION OF THE TUBE AND IS REDUCED WITHIN THE SECOND PORTION THEREOF TO A DIAMETER SOMEWHAT LESS THAN THE DIAMETER OF THE HELIX, SAID FOCUSING MEANS INCLUDING A MAGNET STRUCTURE AND FIELD-STRAIGHTENER MEANS COMPOSED OF DIELECTRIC WASHERS ALTERNATING WITH FERROMAGNETIC WASHERS, THE DISTANCE BETWEEN CONSECUTIVE FERROMAGNETIC WASHERS BEING SMALLER IN THE SAID FIRST PORTION OF THE TUBE THAN IN THE SAID SECOND PORTION THEREOF. 